The present paper contains an analysis of alternative functions to describe chemical equilibrium. In particular we show the equivalence of equilibrium criteria based on pressure, volume and temperature, and corresponding constraints, with respect to those popularly used in thermodynamics, i.e. those based on the minimization of internal energy, enthalpy, Helmholtz and Gibbs potentials. The analysis emphasizes the role of mathematical virtual procedures in determining the equilibrium conditions and the irrelevance of physico-chemical transformations that bring the system into the equilibrium state. Different examples, including a new derivation of Saha's equation, are dealt with to validate this conceptual approach.